It is the idea of belonging to the same big crater, and it has become a telescope after a repair, the Arecibo telescope in the United States.

Arecibo was built in the foothills of Puerto Rico, with a diameter of 305m m m. After the pit was formed, the cable net was erected and then the metal net was laid, and the mirror became (there is a foreshadowing): three towers were erected around, the cable net was pulled to hang the feed, and Arecibo was covered: be careful that such a large pot cannot be moved. But the celestial bodies in the sky are always moving. Therefore, if such an upward cauldron can only look at the opposite direction and the zenith, it will inevitably not meet the requirements of observing astronomical targets. Fortunately, Arecibo uses a spherical primary mirror, so the optical properties of the spherical surface are the same in all directions, and the shapes of the spherical surface and the bottom of the paraboloid are similar in a small range. So we can use different parts of this big pot to observe celestial bodies in different directions: but the approximation of spherical surface and paraboloid is still a little defective. Parabola can focus parallel light on a focus, but spherical surface can't, and it will focus light (electromagnetic wave) on a line: but the approximation between spherical surface and parabola is still a little defective. Parabolic can focus parallel light on a focus, but spherical can't. It will focus the light (electromagnetic wave) on a line: the so-called focal line in the picture. This is the so-called focal line in this picture. In this way, in order to receive the signal distributed on the focal line, the feed must be linear. The so-called feeder: (suspended feeder) (suspended feeder) You can see that there are many holes in the feeder for the collected electromagnetic waves to pass through. The positions of these holes need to be precise. In addition, due to the dispersion of signals, the frequency bandwidth of line-fed energy reception is limited. So people want to know if there is any other way ... Through the design of a clever engineering male, we finally got such a thing ... Through the reflection of the secondary mirror and the tertiary mirror (actually Gregory optical path), Arecibo whose main mirror is spherical can also converge the signal to one point. Arecibo, whose main mirror is spherical, can also converge the signal to one point through the reflection of the secondary mirror and the tertiary mirror (actually Gregory optical path).